1. Field of the Invention
The present invention relates, in general to an optical coupling device and a light-receiving circuit of the same.
2. Description of the Related Art
FIG. 6 is a cross-sectional view of an arrangement for a conventional photocoupler 1. The photocoupler 1 converts an electric signal, which is inputted from a terminal 2 on a primary side, into an optical signal by a light emitting integrated circuit 3 on the primary side, converts the optical signal back into an electric signal by a light-receiving integrated circuit 4 on a secondary side, and outputs the electric signal from a terminal 5 on the secondary side. This electrically isolates a circuit on the primary side from a circuit on the secondary side, thereby realizing sending and receiving of a signal while the devices are electrically insulated from each other. A light emitting element, such as a light emitting diode, on the light emitting integrated circuit 3 and a light-receiving element, such as a photodiode, on the light-receiving integrated circuit 4 are placed in a vicinity to face each other. A gap between the elements is filled with translucent epoxy resin 6 having a given dielectric constant. Further, their outside is sealed with epoxy resin 7 having light blocking effect.
FIG. 7 is a block diagram showing an electrical arrangement for a conventional photocoupler 11. The circuit on the primary side is composed of a sending driver IC 12 and a light emitting element 13, whereas the circuit on the secondary side is composed of a receiving IC 14. In the sending driver IC 12, an amplifier 15 converts a voltage signal, which is inputted to an input terminal IN, into a current signal, and a drive element 16 drives the light emitting element 13 to turn on by using the current signal, where a voltage between terminals Vcc 1 and GND 1 is a power supply voltage. Further, the circuit on the primary side may be composed of only a light emitting element for converting the inputted electronic signal into the optical signal.
In the receiving IC 14, a light-receiving element 17 converts the optical signal into a current signal, where a voltage between terminals Vcc 2 and GND 2 is a power supply voltage. The signal is then converted from current to voltage (hereinafter referred to as I/V conversion) by a current-to-voltage converting amplifier (hereinafter referred to as an I/V converting amplifier) 18, and is subjected to waveform shaping by a comparator 19, and outputted to an output terminal OUT.
Here, a pulse width distortion characteristic is a characteristic for characterizing a photocoupler. Recent FA (Factory Automation) devices may especially have higher performance, for example, and because a semiconductor has higher performance, digital devices are now more widely used. This requires a photocoupler to have a substantially high speed, which insulates between units of an AC servo or a programmable controller for the purpose of reducing noises, and protecting the devices. For example, a photocoupler with a transmission speed of 25 Mbps may have no more than ±6 nsec of the pulse width distortion when the pulse width is 40 nsec.
On the other hand, due to unevenness in a quantity of light outputted by the light emitting element 13, manufacturing unevenness (unevenness caused during a manufacturing process) in distance between the sending-side circuit and the receiving-side circuit caused at a process of molding with the epoxy resins 6 and 7, and the like, a quantity of incoming light into the light-receiving element 17 is substantially changed. Furthermore, there is unevenness in a gain of the I/V converting amplifier 18 caused by manufacturing unevenness of the receiving-side circuit. For realizing a photocoupler having high-speed performance, it is desired that distortion of output pulse width be reduced, which is caused by quantity changes of incoming light into the light-receiving element 17.
Moreover, another characteristic characterizing a photocoupler is a common mode rejection (CMR) ratio. The CMR characteristics indicate how difficult it is to operate by disturbance noise. As shown in FIG. 6, the photocoupler 1 has a condenser structure, in which the epoxy resin 6 having a given dielectric constant is provided between the integrated circuits 3 and 4, that the integrated circuits 3 and 4 are connected by a parasitic capacitor thereof. Accordingly, when the input side and the output 5 side of the photocoupler 1 receive substantially steep noise in which a rising and a falling of the pulse may be represented by the derivative (dv/dt), and a noise current of C.(dv/dt) flows between the input side and the output side, where the parasitic capacitor is C. The noise current may cause the faulty operation, if a part of the noise current flows into the light-receiving element on the light-receiving integrated circuit 4.
One method to prevent the faulty operation covers the light-receiving element with a transparent conductive film such as an ITO film, and its potential is grounded to a GND potential on the receiving side. In such an arrangement, the noise current caused by the parasitic capacitor flows into a GND on the output side via the transparent conductive film, and the light-receiving element receives only the optical signal of the input side. This prevents the faulty operation due to noise, and thus realizing high CMR characteristics. However, this causes a problem that the process becomes complicated because it requires a specialized processing device for forming the conductive film.
Another method to prevent the faulty operation caused by the parasitic capacitor is an arrangement to employ a dummy photodiode, as disclosed in Japanese Patent No. 2531070 (publication date: Sep. 4, 1996), for example. FIG. 8 is a block diagram showing a light-receiving circuit 21 of another conventional art using such a dummy photodiode. The light-receiving circuit 21 is provided with two photodiodes d1 and d2 having identical properties in an identical shape and quantity. Only the photodiode d1 is used for receiving the optical signal from the light emitting element, whereas the other photodiode d2 is shielded from light to be used as a dummy photodiode. The dummy photodiode (12, having its light-receiving face covered with a cathode metal wiring 22, is shielded from light with a cathode potential.
The photodiode d1 and the photodiode d2 are positioned in a cross manner having a checker-board like arrangement, as shown in FIG. 9. In addition, the photodiodes d1 and d2 have an area of approximately 0.1×0.1 mm, which is sufficiently small, whereas frames on which the integrated circuits 3 and 4 are mounted have a size of, for example, 2×2 mm. This makes the noise currents flown into the photodiodes d1 and d2 substantially identical.
Therefore, output currents from the photodiodes d1 and d2 are subjected to the I/V conversion respectively by the I/V converting amplifiers a1 and a2, and compared with each other by a hysteresis comparator 23, which is a differential amplifier, and thus the output from the photodiode d1 is subjected to waveform shaping into a pulse signal. This eliminates a common mode noise component, thereby realizing the output of high CMR characteristics.
However, the I/V converting amplifiers a1 and a2 are amplifiers for linear amplification subjected to negative feedback by resistors r1 and r2 as well as condensers c1 and c2. Therefore, for reducing the distortion of the output pulse width caused by the quantity changes of incoming light, it is desired that a first stage of the amplifier have CL sufficiently wide band. However, there is a problem that the CMR characteristics deteriorate, when the band of the amplifier is wide.
Namely, the photocoupler of high speed and high CMR having the transmission speed of 25 Mbps has an objective to achieve that CMR tolerance is 10 kV/μsec and Vcm=1000V (here, the wording “CMR tolerance” means a level of CMR up to which the photocoupler can tolerate the noise in the common mode noise signal). In this case, as shown in FIG. 10(a), where a rise time of a noise pulse is 100 nsec, its pulse height value is 1 kV. As a result, a noise current waveform flown to the photodiodes d1 and d2 by coupling primary and secondary capacitors has a pulse waveform of 100 nsec, as shown in FIG. 10(b). Therefore, since the noise current waveform includes a high-frequency component of 10 MHz or more, when the band of the amplifier is widened more than a band corresponding to the 25 Mbps, the high-frequency component is easily amplified, and thus easily causing the faulty operation due to noise.
For this reason, the amplifier band cannot be used for obtaining the CMR characteristics, so the quantity of incoming light into the photodiode d1 is required to be constant for obtaining the CMR characteristics. This narrows an allowance for the manufacturing unevenness, thereby causing a problem that a photocoupler of high speed and high CMR is difficult to be manufactured with a sufficient yield.